The present invention relates to a new and improved method for the determination of the trip characteristics of distance relays, especially having zone switching, for protection of lines or networks by means of variable comparison impedances, the measuring element of the distance relay being employed as a comparison instrument.
Distance relays are protection relays which are utilized for the distance protection, e.g. for the time stagger protection of high-voltage networks. As a compound relay they consist of excitation element (response element), measuring element and directional element. They are suitable for the selective determination of two-pole and three-pole short-circuits, double ground faults and single-phase ground shorts or faults. By means of the excitation element the currents and voltages are switched to a measuring element or there is turned-on the measuring function of the relay. Generally, a distance relay possesses a measuring device having an image impedance, the phase angle of which corresponds to that of the line and the magnitude of which corresponds to a certain line length or the distance measuring location-short-circuit location, i.e., the so-called trip or zone boundary or region. The image impedance can be switched by means of stepped or stage timing mechanisms, and specifically, in the direction of increased impedances. This procedure will be explained based upon the illustration of FIG. 1, where there have been shown the trip characteristics of loci curves in the complex impedance plane for different zone settings of a distance relay in the form of circles. Upon the occurrence of a fault or short-circuit in the line to be protected the distance relay is energized. At each relay setting, corresponding to a predetermined trip circle or region, line impedances or appropriate comparison impedances or current and voltage values, which are delivered to the relay at a testing circuit, located within the circle, lead to tripping of the relay. Following a set time t.sub.1 amounting to, for instance, 200 ms, the distance relay is switched to the trip characteristic or region II. Hence, its distance range is enlarged. The same occurs after expiration of the times t.sub.2 and t.sub.3 for the trip characteristics or regions III and IV. The switching operations take place within the relay and normally are not externally influenced.
The loci curves of the distance relay characterize its selectivity upon response to short-circuits, and therefore, must be known or measured, as the case may be, prior to use of the distance relay. The plotting of the loci curves or the trip characteristics or regions of a distance relay was accomplished heretofore by manual operations. By means of a current I and a voltage U there is set an impedance Z and switched to the relay. As a function of the reaction of the measuring element, i.e., its response or nonresponse, there are varied the voltage and/or the current, and thus, the impedance by repeated switching-in operations for such length of time until there was obtained a characteristic point. This type of measuring technique is cumbersome, extremely time-consuming and the operational time is dependent upon the skill of the operator in selecting better impedance values.